The present invention relates to a process for the production of low-chlorine-content polybiphenyl sulfone polymers, to the polybiphenyl sulfone polymers obtainable in this way, to polybiphenyl sulfone polymers with less than 800 ppm content of organically bonded chlorine, to thermoplastic molding compositions and moldings, fibers, films, membranes, or foams comprising the polybiphenyl sulfone polymers mentioned, and also to their use for the production of moldings, of fibers, of films, of membranes, or of foams.
Polybiphenyl sulfone polymers belong to the polyarylene ethers group, and therefore to the class of engineering thermoplastics. Polybiphenyl sulfone polymers have not only high heat resistance but superior notched impact resistance and excellent fire performance, as described by way of example in: E. M. Koch, H.-M. Walter, Kunststoffe 80 (1990) 1146; E. Daring, Kunststoffe 80, (1990) 1149; and N. Inchaurondo-Nehm, Kunststoffe 98, (2008) 190.
The production of polybiphenyl sulfone polymers is disclosed by way of example in DE 1957091 and EP 000361. WO 2000/018824 discloses a process for the production of polybiphenyl sulfone polymers having low content of cyclic oligomers. EP 1272547 describes polybiphenyl sulfone polymers with a particularly low level of intrinsic color, obtained via condensation of the monomers 4,4′-dihydroxybiphenyl and 4,4′-dichlorodiphenyl sulfone in the presence of fine-particle potash.
The prior art usually uses equimolar amounts of the starting materials. However, the resultant content of organically bonded chlorine in the polybiphenyl sulfone polymers from known processes is too high for many applications and often fails to comply with fire-protection requirements. Chlorine contents of less than 1000 ppm are often demanded for applications in the electronics sector, e.g. switches, casings, foils. The known polybiphenyl sulfone polymers moreover have high residual solvent content.
The reaction of the abovementioned monomers in N-methylpyrrolidone (NMP) as solvent is also known per se, for example from EP 0 347 669. NMP has inter alia a number of advantages in terms of process technology. By way of example, the monomers and the potassium carbonate used as base have good solubility in NMP; it is moreover possible to conduct a reaction without any additional entrainer for the water produced by the reaction. Process-technology reasons therefore, make it desirable that the polycondensation reaction for the production of polybiphenyl sulfone polymers is carried out in NMP as solvent.
The abovementioned monomers have extremely high reactivity in NMP as solvent. In many instances when NMP is used as solvent, this generates problems in control of intrinsic viscosity (IV), which characterizes the degree of polymerization.
The person skilled in the art is aware from J. E. McGrath et al., Polymer 25 (1984), 1827 of a method of controlling molecular weight in the condensation of polyarylene sulfones based on bisphenol A. Commercial polyarylene ethers, e.g. Sumika Excel®, have mainly chlorine end groups. No process hitherto disclosed produces polybiphenyl sulfone polymers in NMP by using an excess of the aromatic dihydroxy compound.